Currently, a power supply device is usually for maintaining a variety of power consumption equipments to continue to work for a period of time after being powered off such that the user can process the emergency, for example, in the computer field, a user can timely save data such that the work will not be affected or the data will not be lost. In various actual applications in a computer system, a network system and a communication system, a power supply device can be used as an emergency power supply to avoid interrupting normal work due to sudden power off and causing damage to the systems.
As various electronic and electric power products are widely used, power source harmonic wave problem and power factor worsening problem tend to be more and more serious. In order to improve power supply quality and service efficiency of energy sources, all the organizations formulate criteria for harmonic waves and power factors generated by electronic and electric power products. The two methods for improving power factors are mainly divided into a passive one and an active one. Just as its name suggests, the passive method means only passive elements are used in circuits to improve power factors as inductance, transformer or the like. The method has advantages as, less electromagnetic interferences, simple design and etc., but it has a limited effect in power factor improvement. An active power factor correction circuit means using active elements, such as, a power element, a diode or the like. It can both adjust output voltage and drastically improve power factor. However, it requires a well cooperated control due to the increase of circuit complexity.
As an inverter works at a high frequently switching state, the APFC technique has advantages as, small size, small weight, high efficiency and power factor being close to one and the like. The basic idea for APFC is: making a full-wave rectification of input alternating voltage, adding a DC/DC conversion between a rectification circuit and a filtering capacitor, enabling, by a proper control (i.e. Pulse-Width Modulation), waveforms of input currents to automatically follow waveforms of input voltages, that is, correcting the input currents as sine waves in phase with the input voltage and making input impedance purely resistive, thus to achieve stable voltage output and unit power factor input and increase PF approximately to 1.0. APFC technique adapts to the development orientation for electric power and electronic techniques.
The widely used off-line power supply devices at present, such as, an off-line uninterrupted power supply (UPS), are used in various fields. When a commercial power exists, the charger of an off-line power supply device charges a storage battery and floating charges. When a commercial power is off, output conversion switch of the off-line power supply device disconnects with the grid and connects with an inverter and continues to supply power to loads. At this time, the inverter works to change direct voltages (supplied by a battery) into alternating voltages meeting requirements for loads.
FIG. 1 is a schematic diagram illustrating a square wave power supply device in the prior art. Said square wave power supply device comprises a relay, a battery and an inverter. When the commercial power works normally, the relay maintains normal connection with the commercial power line I/P-L and supplies power to loads via commercial power input line I/P-L and I/P-N. Meanwhile, the commercial power charges the battery in the square wave power supply device while supplying power normally such that the battery can supply power to the loads when the commercial power malfunctions. When the commercial power is down or malfunctions, the relay disconnects the connection with the commercial power line I/P-L and connects with the output INV-L of the relay. The battery outputs electric energy to the inverter, and the inverter outputs square wave and supplies power to loads via lines INV-L and INV-N. Output of the inverter is as shown by square wave waveform of INV O/P.
FIG. 2 is a schematic diagram illustrating another square wave power supply device in the prior art. Said square wave power supply device comprises a relay, a battery, an inverter and a transformer. When the commercial power works normally, the relay maintains normal connection with the commercial power line I/P-L and supplies power to loads via commercial power input line I/P-L and I/P-N. Meanwhile, the commercial power charges the battery in the square wave power supply device while supplying power normally such that the battery can supply power to the loads when the commercial power malfunctions. When the commercial power is down or malfunctions, the relay disconnects the connection with the commercial power line I/P-L and connects with the output INV-L of the relay. The battery outputs electric energy to the inverter which adjusts voltage via the transformer and outputs square wave, and supplies power to loads via INV-L and IINV-N. Output of the inverter is as shown by square wave waveform of INV O/P.
However, in the prior art, usually there will be the following problems in the initial cycles (e.g. the first cycle) after switching from a commercial power mode to a battery mode:    1. The voltage effective value is too low in the first cycle. In the first cycle after the existing square wave UPS switches to a battery mode, output voltage outputs a square wave width by setting a fixed criterion. When APFC load switches to a battery mode, the actual output voltage will be lowered by peak voltage, resulting in that the effective output voltage is far smaller than the rated voltage and this cannot well support APFC power supply in work.    2. Pulse width is too small in the first cycle. Currently, in the first cycle after the square wave UPS switches to a battery mode, output voltage zero point may takes commercial power zero point as reference. When commercial power is off at different angles, the actual open widths of output wave are different and output voltages differ greatly, wherein, when commercial power is off at some angles, UPS effective output voltages are apparently low, which cannot well support APFC power supply in work during APFC LOAD conversion.
In addition, in a battery mode, the problem of output voltage effective value being too low will appear during loads increase, take on loads or APFC load conversion. When the current square wave UPS is in a battery mode, the output voltage peak will be lowered greatly if loads increase drastically or APFC Load is activated, or UPS APFC load converts. It, needs 2-3 cycles to increase the output voltage wave width to the maximum, and voltage recovery time is long. During APFC LOAD, the output voltage being continuously low causes APFC LOAD to work abnormally.
The principle for output voltage generation of a square wave machine in the prior art is: output voltage peak of square wave UPS in a battery mode decreases as the battery voltage deceases and load increases, so in order to obtain an expected effective output voltage, the waveform width of an output square wave voltage needs to be adjusted.